The stages are decelerating all the way down to the ground, such that their velocity reaches (almost) 0 m/s at ground level, at which point they shut down. Once they slow down enough, it looks like they're descending at a constant speed (especially from a distance), but they're decelerating the whole way. They cannot hover - if they don't shut the engines down when velocity is 0 m/s, they'll start rising again.
– John BodeApr 20 at 18:48

2 Answers
2

The falcon 9 first stage cannot hover as the thrust of one Merlin engine even at its lowest thrust is able to overcome the mass of the almost-empty first stage. The stage appears to decelerate very slowly in the final moments of the landing as a result of the engine firing pattern SpaceX uses.

Per Elon Musk:

Thanks! 3 of 9 engines are lit initially, dropping to 1 near ground. Even w 1 lit, it can't hover, so always land at high g
- Elon Musk (@elonmusk) April 15, 2015

So as soon as the 3 engines are fired, the stage decelerates rapidly, but when the two outer ring engines are shut down for final approach, the deceleration of the stage slows rapidly, giving the illusion that it is hovering. Although this tweet is from early 2015, we know that the engines on the Falcon 9 have only gotten stronger, so it would have no chance of hovering. In order to land, SpaceX uses a hover-slam or suicide burn, so as soon as the stage reaches zero velocity while decelerating, it’s on the ground/ASDS. Blue Origin’s New Shepard rocket, however, can hover above the ground and does so to travel laterally before landing on the pad.

didn't address the apparent lack of deceleration in the last stage of flight, and whilw acknoleging the engines have changed your answer is essentialy "the old ones couldn't so the new ones wont either"
– JCRMApr 20 at 1:14

@JCRM Yes that is exactly what I’m saying, couldn’t say it better than that.
– Jake BlockerApr 20 at 1:15

3

The Falcon Heavy boosters ignited their landing burns lower than past RTLS flights, braked hard on three engines, and then did the final landing on single engines at a more leisurely pace. They remain, as far as we know, incapable of hovering. They did show a very obvious reduction in braking speed during the end of the Heavy landing, though.
– SaibooguApr 20 at 2:58

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I find the "is able" in the first sentence confusing. I believe you're saying that the engine can't throttle low enough to maintain parity with the weight of the booster, which means that once the downward momentum is negated, the booster will want to go back up again. However, "is able" could be a typo: "is unable" implying that it can't throttle high enough to stay aloft. I think the opening sentence could be improved if this was clarified.
– Roger LipscombeApr 20 at 9:35

2

Yeah, I know it's not a typo; it's just that I had to read it a couple of times to figure that out. My English grammar is stronger than my rocket science.
– Roger LipscombeApr 20 at 13:52

The Merlin 1D claims to be able to throttle to 40%, which would be about 350 kN thrust at sea level. The dry mass of a Falcon 9 first stage is about 24 tons, so it can't hover, but it can get pretty close. Thrust to weight would be about 1.3.
All data from wikipedia.